Aminotransferase and the production of alanine during hyperosmotic stress in Paramecium calkinsi

Osmoregulation inParamecium: in situ ion gradients permit water to cascade through the cytosol to the contractile vacuole

In vivo K+, Na+, Ca2+ and Cl-activities in the cytosol and the contractile vacuole fluid of Paramecium multimicronucleatum were determined in cells adapted to a number of external osmolarities and ionic conditions by using ion-selective microelectrodes. It was found that: (1) under standardized saline conditions K+ and Cl- were the major osmolytes in both the cytosol and the contractile vacuole fluid; and (2) the osmolarity of the contractile vacuole fluid, determined from K+ and Cl- activities only, was always more than 1.5 times higher than that of the cytosol. These findings indicate that excess cytosolic water crosses the contractile vacuole complex membrane osmotically. Substitution of choline or Ca2+ for K+ in the external solution or the external application of furosemide caused concomitant decreases in the cytosolic K+ and Cl- activities that were accompanied by a decrease in the water segregation activity of the contractile vacuole complex. This implies that the cytosolic K+ and Cl- are actively coimported across the plasma membrane. Thus, the osmotic gradients across both the plasma membrane and the membrane of the contractile vacuole complex ensure a controlled cascade of water flow through the cell that can provide for osmoregulation as well as the possible extrusion of metabolic waste by the contractile vacuole complex.

Developmental regulation of proline transport in Leishmania donovani

Leishmania donovani are the causative agents of kala azar in humans. These organisms cycle between the proline-rich environment of the sand fly vector (extracellular promastigotes) and the sugar-rich condition in the mammalian host (intracellular amastigotes). Parasites have adapted to these extreme changes in proline concentrations: promastigotes utilize proline as a carbon source, whereas amastigotes utilize sugars and fatty acids. Previous studies have suggested that promastigotes and amastigotes express distinct proline transporters. However, the information available on these transporters is limited. In this work, proline transport was investigated in axenic L. donovani cultures. Three transport systems were identified: cation-dependent and -independent proline transporters in promastigotes (systems A and B, respectively) and a single cation-independent transporter in amastigotes (system C). Systems A and C have broad specificity to almost all amino acids and obtain optimum activity at acidic pH ranges (pH 6 and 5, respectively). System B is more specific to proline, as it is inhibited by only five amino acids. Temperature response analyses indicated that the transporters of both promastigotes and amastigotes perform best at 37 degrees C. The activity of system A during parasite differentiation was assessed. The transport activity of system A disappeared 3 days after promastigotes were induced to differentiate into amastigotes. In these cells, elevated temperature and acidic pH each suppressed the activity of system A. When amastigotes were induced to differentiate back into promastigotes, system A resumed its activity 24 h after differentiation was initiated. In conclusion, L. donovani obtain proline transport systems that are stage specific, regulated by both pH and temperature. This paper constitutes the first investigation of amino acid transport in axenic L. donovani.

Osmotic homeostasis in Dictyostelium discoideum: excretion of amino acids and ingested solutes

The response to osmotic stress in axenically cultured Dictyostelium discoideum was examined. Hypoosmotic buffers elicited two changes in the large (approximately 50 mM) cytosolic pool of amino acids: a) the total size of the pool diminished, while b) about half of the initial pool was excreted. Hyperosmotic stress had the opposite effect. Among the predominant amino acids in the pool were glycine, alanine and proline. Putrescine, the major diamine, was neither excreted nor modulated. Recently ingested radioactive amino acids were excreted in preference to those in the cytoplasm, suggesting that the endocytic pathway might be involved in water excretion. Furthermore, hypoosmotic stress stimulated the selective excretion of small, membrane-impermeable fluorescent dyes which had been ingested into endocytic vacuoles. Caffeine inhibited the excretion of the fluorophores but not the amino acids. We conclude that the response of Dictyostelium to osmotic stress is complex and includes both modulation of the cytoplasmic amino acid pool and the excretion of amino acids and other small solutes from the endocytic pathway.